1. Field of the Invention
The present invention relates to touch panel technology and more particularly, to a multipoint sensing method for capacitive touch panel.
2. Description of the Related Art
A touch panel technology of capacitive type is capable of recognizing the touch of a human body by means of detecting the variation of capacitance. The touch panel is normally comprised of electrode arrays and a capacitive sensing circuit. When sensing signals are inputted onto a top electrode layer and a bottom electrode layer of the electrode arrays, the capacitive sensing circuit detects the specific capacitance of a plurality of capacitors which are formed between the two electrode layers and located at each intersection crossed by row electrodes and column electrodes patterned respectively on the two electrode layers. Therefore, the capacitive sensing circuit can judge whether any human touch happened on the touch panel to cause an equivalent change of the capacitance during the process of reading the capacitance at every intersection.
While determining the touch point and reading the relatively capacitance, the capacitive sensing circuit needs to obtain the current flows of the electrodes across the respective touch point and to measure the respective capacitance after the electrodes have been recharged to stable charge distribution. If the touch panel has a big size or needs to proceed the detection of multiple touch points, the parasitic resistance presenting in the current path may affect the detection signals for determination of the touch point, extend the charging time of the electrode capacitance and thereby causing an extension of the response time of the capacitive sensing circuit, and give rise to noise interference with the detection signals along the charge transfer path as well as decrease the S/N ratio (signal-to-noise ratio) of the detection signals measured by the capacitive sensing circuit. Taking an example as regular transparent electrodes applied for conducting wires of touch panel and made from ITO (indium-tin oxide) material, which having a higher resistive component than other regular metal wires and with a relatively increasing rate of the resistance subject to increase the touch panel size, the resistive factor of the ITO material results in low response speed of the capacitive sensing circuit, and additional sensing circuits will be required if it is necessary for accelerating the response speed. However, increasing the number of sensing circuits greatly increases the circuit installation space and manufacturing cost. In case of using a different electrode material to decrease the resistance in the charge transfer path, electrode materials currently available on the market do not have as high transmittance for visible light as ITO electrodes to maintain light transparency required for the touch panel.
US 2007/0257890A1 discloses a controller for touch panel, which needs to provide an ASIC (Application Specific Integrated Circuit) to interface with a digital signal processor (DSP) in executing multipoint sensing. To solve the problems resulted from the parasitic resistance existing in the transparent electrode material, the ASIC includes detecting signals of multiple waveforms outputted with different frequencies for noise-free signal can be detected, a demodulation circuit for filtering noise, and a time delay circuit to match with delayed capacitive response time so as to raise the sensing accuracy. The ASIC further includes charge amplify means, charge conversion means and static capacitance offset means. Therefore, the circuit structure of this ASIC is complicate and expensive to manufacture, increasing the manufacturing cost of the touch panel.
U.S. Pat. No. 6,466,036B1 provides a logic control circuit with sensing capacitors and multiple switching devices coupled with the capacitors to have the charge variation related to the touching event on a touch panel be directly converted into a logic output for determination of touch point. This technique simplifies the circuit structure and saves much time in waiting for charge re-distribution of the capacitance. However, when this technique is used for multiple touch points application, more number of switch devices and sensing capacitors must be provided, increasing circuit installation space and manufacturing cost as well.